A particular application of spherical surfaces is in coordinate measuring machines (CMMs) or metrology tips, where a spherical tip is mounted on a stem and used to measure or profile a workpiece. In metrology, spherical tips are used to map out the shape and roughness of surfaces made from metals, glasses, ceramics, crystalline, and other materials, whose surface shape needs to be measured with high accuracy. The tip is usually mounted on a partially flexible arm, which is fitted with a highly sensitive detector for sensing any flexing of the arm. Typically, movements of the order of a few nanometres can be detected. At the start of a measurement the tip is brought in contact with the object to be measured. Usually either the arm with the tip or the object to be measured itself is mounted on a XYZ translation stage and/or a rotary stage and the tip or the object is displaced or rotated according to a pre-programmed motion. Deviations of the shape of the object from the shape described by the pre-programmed motion are then detected by the flexing of the arm on which the tip is mounted.
In typical metrology applications, since the object to be measured is in general non-planar, different parts of the tip are in contact with the measurement object. In order to have a flexing of the measurement arm which is independent of the position on the tip, which is in contact with the object, the tip itself needs to be spherical to an accuracy which exceeds the accuracy of the motion so that the accuracy of the measurement is not adversely affected by the shape of the tip. Currently available tips suffer from the problem of wear, since the tip can be in continuous sliding contact with the surface of the object. Especially when hard and/or rough materials are measured, the wear of the tips leads to rapid deterioration of the spherical shape of the tip. This leads to measurement errors when using these tips. In such cases, the tip must be replaced by a new and undamaged tip. This leads to a high cost of measurement caused by the high cost of the tips and the need to recalibrate each new tip. Another common problem with softer materials such as aluminium is a build-up, even during a single measurement, of the material from which the measurement object is made on the surface of the tip, thus leading to measurement errors.
Furthermore in metrology one may want to measure the size of a hole in a material by passing balls of different diameters through the hole and determining the maximum size ball that does pass through the hole. In that case it is important that the balls are exactly spherical, do not deform and have low wear characteristics. For this application a completely spherical ball is not necessarily required, instead a super-hemispherical surface may suffice.
Computer operated machines which encompass such probes are increasingly replacing the use of traditional manually operated measuring instruments such as vernier callipers, micrometers and dedicated gauges. Such machines are used to assess the quality of material surfaces in many different fields. A field where quality control is of particular importance is in the automotive industry e.g. for the measuring of cylinder bore diameters on engine blocks. The tips can be used in a number of modes, including ‘touch and step’ (or ‘touch mode’), and continual profiling (or ‘scanning mode’).
An example of such a machine is a coordinate measuring machine (CMM). CMM probes are transducers which convert physical measurements into electrical signals. The most common of these probes is the “touch trigger” probe. This works by actually contacting the surface of the object which is to be quality controlled. Upon contact with the surface, a signal with the coordinates of that point is sent to the CMM.
An alternative type of CMM probe is the scanning probe. This is passed across the surface of the target surface and transmits a continuous flow of data to the measurement system.
In both types of CMM probe, it is important that the probe itself has the correct properties. In order to give an accurate measurement which is, in fact, representative of the quality of the surface under observation, the probe should be spherical, resistant to wear and have a surface with a low roughness Ra or Rq (where Ra is the average roughness and Rq is the root mean square (RMS) roughness, both terms well known in the art).
Conventionally, CMM probes are typically made from high chrome, high carbon, stainless steel. In evaluating the failure of CMM tips, particularly the case of profiling tips, three typical failure methods have been identified:
1. As the tip slides over the surface fragments of aluminium stick to the tip material. In this phenomenon, detritus from the surface under observation accumulates on the tip of the probe leading to a degradation in performance. This is particularly the case where the surface under observation is formed from aluminium, as is common in the automotive industry;2. The sliding contact wears a flat onto the tip (wear). Metrology tool tips and balls are currently made from hard materials such as tungsten carbide, ruby or sapphire but even these materials show wear over time;3. On some surfaces the friction causes stick-slip motion of the tip, decreasing the accuracy of the measurement. To ensure accuracy, the probe must move over the surface under observation in a continuous manner rather than in a jolting motion.
Of these three failure methods, often the dominant one is not wear, but is accumulation of material from the surface being profiled, for example Al from an Al surface. As such, it is not obvious that particularly tough materials offer any real advantage over materials such as ruby, which is comparatively cheap, available in large solid pieces, and easy to fabricate to shape.
There is thus a great need for tool tips which are resistant to wear, resistant to the accumulation of detritus thereon and which can be caused to move over the surface under observation in a reliable continuous motion.